Question

What element has a ground state electronic configuration of `[Kr]5s^2 4d^5`?

Answer 1

That would be technetium, element 43.

Explanation:

When dealing with the periodic table and electron structures, it's good to know the atomic numbers of the noble gases:

Helium = 2
Neon = 10
Argon = 18
Krypton = 36
Xenon = 54
Radon = 86

Oganesson = 118 (This is a name designated for the newly discovered seventh noble gas.)

Here the noble gas core is ktypton, element 36, so we have 36 electrons plus the two 5s and five 5d electrons that are written out. Thus 43 electrons, so the neutral atom has atomic number 43. Look at a periodic table and you see that this element is technetium.

Technetium is so named because it was once thought to be all artificial. Today we know that traces of technetium do occur in nature, but no isotopes are stable. See https://en.wikipedia.org/wiki/Technetium.